Thermally insensitive particle concentration controller

ABSTRACT

An apparatus in which the concentration of particles in a mix is controlled substantially about a preselected ratio. An electrode passes through the mix and attracts particles thereto. The electrode, with the particles adhering thereto, is illuminated, and the light rays transmitted therefrom are detected by a substantially thermally insensitive sensor. The electrical output signal from the sensor is stably amplified to at least a predetermined level. The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

United States Patent Reyner THERMALLY INSENSITIVE PARTICLE CONCENTRATION CONTROLLER Noel L. Reyner, Hilton, N.Y.

Xerox Corporation, Stamford, Conn.

Nov. 5, 1973 Inventor:

Assignee:

Filed:

Appl. No.:

US. Cl. 222/56; 118/637; 222/DIG. 1; 355/3 DD Int. Cl. G03G 15/08 Field of Search 222/56, 57, 52, DIG. 1; 307/252 J; 118/637; 355/3 R, 3 DD [56] References Cited UNITED STATES PATENTS 4/1968 Kamola 222/DIG. 1 9/1969 l-larbaugh 307/252 JX 1/1970 Gawron 222/DIG. 1

OTHER PUBLICATIONS RCA Institutes, Integrated Circuits, 1969, p. 8.07.

[ Dec. 16, 1975 Primary Examiner-Robert B. Reeves Assistant ExaminerJoseph .l. Rolla Attorney, Agent, or FirmI-l. Fleischer; C. A. Green; J. J. Ralabate An apparatus in which the concentration of particles in a mix is controlled substantially about a preselected ratio. An electrode passes through the mix and attracts particles thereto. The electrode, with the particles adhering thereto, is illuminated, and the light rays transmitted therefrom are detected by a substantially thermally insensitive sensor. The electrical output signal from the sensor is stably amplified to at least a predetermined level.

The foregoing abstract is neither intended to define the invention disclosed in the specification, nor is it intended to be limiting as to the scope of the invention in any way.

ABSTRACT 14 Claims, 6 Drawing Figures in :7 I06 72 a2 w fi 1k 2 88 at.

100 I00 100 65 t x" m 90 9a 9a 9a 104 Kb 80 9s 9s 96 84 I US. Patent Dec. 16,1975 Sheet10f4 3,926,338

we x F/G I US. Patent Dec.16,1975 Sheet20f4 3,926,338

U.S. Patent Dec. 16, 1975 Sheet30f4 3,926,338

US. Patent Dec.16, 1975 Sheet40f4 3,926,338

% HQK THERMALLY INSENSITIVE PARTICLE CONCENTRATION CONTROLLER BACKGROUND OF THE INVENTION This invention relates generally to a multi-color electrophotographic printing machine, and more particularly concerns an apparatus which controls the concentration of toner particles in a developer mix employed therein.

In the process of electrophotographic printing, a charged photoconductive surface is irradiated to create an electrostatic latent image corresponding to an original document. The electrostatic latent image is developed with toner particles from a developer mix. Thereafter, the toner particles are transferred to a sheet of support material and permanently affixed thereto in image configuration. In the foregoing manner, a copy of the original document is created.

This process is not limited to black and white copies but may be employed with color copies in a similar manner. In color reproduction, a plurality of successive single color toner powder images are transferred to the sheet of support material in superimposed registration with one another. Each image corresponds to a filtered light image of the original document. Thus, the multilayered toner powder image, when permanently affixed to the sheet of support material, corresponds in color to the original document. In order to insure that the toner powder image is satisfactory, developability must be controlled. Developability is related to the concentration of toner particles in the developer mix, ie the percentage of toner particles relative to carrier granules in the developer mix.

Various prior art systems have been devised to regulate the concentration of toner particles in the devel oper mix. One such system is described in US. Pat. No. 3,399,652 issued to Gawron in 1968. This patent discloses a rotating reflective disc positioned in the developer mix. The disc is electrically biased to attract toner particles from the developer mix. A light beam is reflected from the surface of the disc and transmitted to a photoelectric unit. The intensity of the light rays received by the photoelectric unit is indicative of the density of toner particles adhering to the disc surface. An electrical output signal from the photoelectric unit initiates dispensing of toner particles to the developer mix to replenish the supply thereof.

Another patent of interest is US. Pat. No. 3,754,821 issued to Whited in 1973. The apparatus disclosed therein is a transparent electrode mounted on a photoconductive drum. As the electrostatic latent image on the photoconductive drum is developed, the transparent electrode is biased electrically to attract toner particles thereto. The transparent electrode, with the toner particles adhering thereto, is illuminated and the light rays transmitted therethrough are detected by a photosensor mounted in a thermal chamber. The light rays are transmitted to the photosensor by fiber optics dis- 7 posed between the photosensor and the transparent electrode. Utilization of a' thermal chamber and the fiber optics in the foregoing apparatus substantially increases the cost and difficulties of manufacture. However, this arrangement is required in order to com: pensate for the thermal sensitivity of the photosensor. For example, a phototransistor has a sensitivity of l.5%/C, and a photodarlington has a sensitivity. of 3.5%/C. The foregoing temperature sensitivity exceeds the allowable system tolerance. Temperature sensitivities of this magnitude would result in the system detecting variations in temperature rather than variations in toner particle density. One type of photosensor which has a sufficiently low thermal sensitivity is a photodiode. A photodiode has a sensitivity of 0.2%/C. However, at the light levels involved, the current generated by a photodiode would be extremely low.

Accordingly, it is the primary object of the present invention to improve the apparatus utilized to regulate the concentration of toner particles within a developer mix by providing a substantially thermally insensitive photosensor cooperating electrically with a stable amplification circuit.

SUMMARY OF THE INVENTION Briefly stated, and in accordance with the present invention there is provided an apparatus for controlling the concentration of particles in a mix.

Pursuant to the present invention, the apparatus includes electrode means, illuminating means, sensing means and amplifying circuit means. In operation, the electrode means is biased electrically to attract particles thereto when passing through the mix. Thereafter, light rays from the illuminating means irradiate the electrode with the particles adhering thereto. Sensing means detect the intensity of the light rays transmitted Y from the electrode means and develop a substantially thermally insensitive electrical output signal. This electrical output signal is stably amplified to at least a preselected level by the amplifying circuit means. In this manner, the concentration of particles within the mix is determined and may thereupon be controlled to a desired ratio.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawing, in which:

FIG. 1 is a schematic perspective view of a multicolor electrophotographic printing machine embodying the teachings of the present invention therein;

FIG. 2 is a sectional elevational view of the photoconductive drum employed in the FIG. 1 printing machine;

FIG. 3 is an electrical diagram illustrating the amplifying circuit of the present invention;

FIG. 4 is an electrical diagram depicting the comparator circuit of the present invention;

FIG. 5 is an electrical diagram showing the hold circuit of the present invention; and

FIG. 6 is an electrical diagram depicting the circuitry of FIGS. 3, 4 and 5 electrically connected to one another.

While the present invention will be described in connection with a preferred embodiment thereof, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION With continued reference to the drawings wherein like reference numerals have been used throughout to designate like elements, FIG. 1 schematically illustrates 3 an electrophotographic printing machine arranged to produce multi-color copies from a colored original document. Although the control apparatus of the present invention is particularly well adapted for use in the electrophotographic printing machine depicted in FIG. 1, it should become evident from the following discussion that it is equally well suited for use in a wide variety of electrostatographic printing machines and is not necessarily limited to the particular embodiment shown herein. 1 The printing machine depicted in FIG. 1 employs a drum 10 having a photoconductive surface 12 mounted thereon and entrained about the exterior circumferential surface thereof. Photoconductive surface 12, preferably, is manufactured from a material having a relatively panchromatic response to white light. For example, one type of suitable photoconductive material is disclosed in US. Pat. No. 3,655,377, issued to Sechak in 1972. Drum 10 is mounted rotatably within the machine frame and adapted to rotate in the direction of arrow 14. A series of processing stations are positioned such that as drum 10 rotates in the direction of arrow 14, photoconductive surface 12 passes sequentially therethrough. Drum 10 is driven at a predetermined speed by a drive motor (not shown) relative to the various machine operating mechanisms. The timing disc mounted in the region of one end portion of the shaft on which drum 10 is disposed cooperates with the machine logic to coordinate each operation at the respective stations producing the proper sequence of events thereat. Initially, drum l advances photoconductive surface 12 through charging station A. Charging station A has positioned thereat a corona generating device, indicated generally by the reference numeral 16. Corona generating device 16 extends in a generally transverse direction across photoconductive surface 12. In this manner, corona generating device 16 is readily able to charge photoconductive surface 12 to a relatively high substantially uniform potential. A suitable corona generating device is described in US. Pat. No. 2,778,946 issued to Mayo in 195 7.

After photoconductive surface 12 is charged to a substantially uniform level, drum rotates to exposure station B. At exposure station B, a moving lens system, generally designated by the reference numeral 18, and a color filter mechanism, shown generally at 20, create a color filtered light image which irradiates charged photoconductive surface 12. An original document 22 is stationarily supported face down upon transparent viewing platen 24. Original document 22 may be i Y a sheet of paper, book or the like which is disposed upon platen 24. Lamp assembly 26, filter mechanism 20 and lens 18 move in a timed relation with drum 10 to scan successive incremental areas of original document 22 disposed upon platen 24. During exposure, filter mechanism 20 interposes selected color filters into the optical path of lens 18. A selected color filter operates on the light rays passing through lens 18 to create a single color light image. The single color light image irradiates charged photoconductive surface 12 i three individual developer units generally designated by the reference numerals 28, 30 and 32, respectively. A suitable development station employing a plurality of developer units is disclosed in copending application Ser. No. 255,259 filed in I972 now US. Pat. No. 3,854,449. The development units are all of a type generally referred to as magnetic brush developer units. A typical magnetic brush developer unit employs a magnetizable developer mix comprising carrier granules and toner particles. The developer mix is continually brought through a direction flux field to form a brush thereof. The developer mix is continually moving to provide the brush consistently with fresh material. Development is achieved by bringing the brush into contact with photoconductive surface 12. Each of the developer units 28, 30, 32 apply toner particles corresponding to the complement of the specific single color electrostatic latent image recorded on photoconductive surface 12. The toner particles are adapted to absorb light within a preselected spectral region of the electromagnetic wave spectrum corresponding to the wave length of light transmitted through the filter. For example, a green filtered electrostatic image is rendered visible by depositing green absorbing magenta toner particles thereon. Similarly, blue and red latent images are developed with yellow and cyan toner particles, respectively.

The control apparatus of the present invention includes a transparent electrode assembly 46 mounted on photoconductive surface 12 of drum 10. Illuminating means, such as light source 48, in cooperation with fiber optics 50, transmit light rays through transparent electrode assembly 46. During development, toner particles are deposited on transparent electrode 46 and the intensity of the light rays passing therethrough is indicative of the density thereof. Sensing means, such as photodiode 52, is adapted to receive the light rays transmitted through transparent electrode assembly 46. Photodiode 52 develops an electrical output signal corresponding to the intensity of the light rays received thereon. Electrical circuitry 54 processes the electrical output signal from photodiode 52. It should be noted that photodiode 52 is substantially thermally insensitive having a coefficient of thermal sensitivity of about O.2%/C in the mode presently being used. Electrical circuitry 54 is adapted to amplify the electrical output signal from photodiode 52 which is at a lowlevel for the intensity of light rays transmitted thereto. In addition, electrical circuitry 54 compares the amplified electrical output signal to a preselected reference and develops an error signal corresponding to the deviation therebetween. Finally, electrical circuitry 54 includes a hold circuit adapted to hold the error signal until it is interrogated by the logic associated with one of the respective toner dispensers 34, 36 or 38. Thus, the electrical output signal from electrical circuitry 54 is adapted to energize the corresponding toner dispenser to adjust the concentration of toner particles within the associate developer mix to a preselected level optimizing development thereby. The detailed structural configuration of the circuitry employed in electrical circuit 54 will be discussed hereinafter with reference to FIGS. 2through 6, inclusive. The foregoing control apparatus is disclosed in greater detail in US. Pat. No. 3,754,821 issued to Whited in 1973, the disclosure of which is hereby incorporated into the present application.

After development, drum 10 is rotated to transfer station D. At transfer station D, the toner powder,

roll shown generally at 58. The surface of transfer roll 58 is biased electrically to a potential having a magnitude and polarity sufficient to electrostatically attract toner particles from photoconductive surface 12 to support material 56. A single sheet of support material 56 is supported on transfer roll 58 which is arranged to move in synchronism with photoconductive surface 12 (at substantially the same angular velocity therewith) so that each of the developed images is placed in superimposed registration upon support material 56.

Prior to proceeding with the remainder of the processes in the electrophotographic printing machine, the sheet feeding arrangement will be briefly discussed. Support material 56 is advanced from a stack thereof. A feed roll, in operative communication with a retard roll, advances and separates the uppermost sheet from the stack. The advancing sheet moves into a chute which directs it into the nip between a pair of register rolls. Thereafter, gripper finger, mounted on transfer roll 58, secure releasably thereon support material 56 for movement in a recirculating path therewith.

After a plurality of toner powder images have been transferred to support material 56, the gripper fingers on transfer roll 58 release support material 56 and space it therefrom. A stripper bar is then interposed therebetween to separate support material 56 from transfer roll 58. Subsequently, an endless belt conveyor advances support material 56 to fixing station E.

At fixing station E, a suitable fuser permanently affixes the transferred multi-layered toner powder image to support material 56. One type of suitable fuser is described in U.S. Pat. No. 3,498,592 issued to Moser et al. in 1970. After the fixing process, support material 56 is advanced by a plurality of endless belt conveyors to a catch tray for subsequent removal from the machine by an operator.

The last.processing station in the direction of drum rotation, as indicated by arrow 14, is cleaning station F. Although a preponderance of the toner particles are transferred to support material 56, residual toner particles remain on photoconductive surface 12 after transfer of the powder image therefrom. These residual toner particles are removed from photoconductive surface 12 as it passes through cleaning station F. Here, the residual toner particles are initially brought under the influence of a cleaning corona generating device (not shown) adapted to neutralize the electrostatic charge remaining on photoconductive surface 12 and the residual toner particles adhering thereto. The neutralized toner particles are then cleaned from photoconductive surface 12 by a rotatably mounted fibrous brush 60 in contact therewith. A suitable brush cleaning device is described in U.S. Pat. No. 3,590,412 issued to Gerbasi in 1971. In this manner, residual toner particles remaining on photoconductive surface 12 are readily removed therefrom.

Referring now to FIG. 2, drum is illustrated therein with transparent electrode assembly 46 mounted thereon. Electrode assembly 46 is located in a non-image portion of photoconductive surface 12. As electrode assembly 46 passes through the development zone, the conductive surface thereof is biased to an electrical potential simulating the electrostatic latent image on photoconductive surface 12 of drim 10. Preferably, the electrode is biased to about 200 volts above developer bias, normal developer bias being about 500 volts. However, the electrode may be biased from about 100 volts to about 600 volts above the developer bias. The density of toner particles on transparent electrode 46 is sensed by photodiode 52. The output signal from photodiode 52 is processed by electrical circuitry 54 and, depending upon the density of toner particles remaining on electrode 46, additional toner particles may or may not be furnished to the developer mix. Photodiode 52 is mounted exterior to and spaced from photoconductive surface 12 of drum 10 to sense the intensity of light rays passing through transparent electrode 46 just prior to the cleaning of photoconductive surface 12, i.e. before drum 10 is rotated to cleaning station E since electrode 46 undergoes the normal photoconductive drum cleaning process. Light source 48 may be inside drum 10, or as shown in FIG. 2, external to drum 10 with the light rays conducted therein by means of fiber optics 62. Shaft 64, which supports drum 10, is a tubular member and permits fiber optics 62 to pass through the hollow central core thereof and out therefrom to photoconductive surface 12. This enables light rays from light source 48 to be directed to transparent electrode assembly 46.

In order to apply the appropriate voltage corresponding to the electrostatic latent image recorded on photoconductive surface 12 of drum 10, transparent electrode assembly 46 is biased to a suitable voltage level. Preferably, this is achieved by mounting a commutator assembly, indicated generally at 66, in the region of the end bell of drum 10. A suitable slip ring assembly may be used in lieu of commutator assembly 66. The timing of the application of the bias voltage to an electrode assembly 46 may be controlled by suitable electronic switching or by the use of a split commutator ring, i.e. the electrode being biased over one portion of the commutator and not over the remaining portion. The bias voltage is removed from transparent electrode assembly 46 during the cleaning process. In lieu of applying a bias voltage to transparent electrode assembly 46, a suitable bias may be applied thereto by electrical charging.

Turning now to FIG. 3, there is shown the amplifying circuit means 68 of electrical circuitry 54. Amplifying circuit means 68 is electrically connected to photodiode 52. Operational amplifier 70 thereof is electrically connected to photodiode 52. A 100,000 ohm resistor 72 and a 0.01 microfarad capacitor 74 are connected in parallel with operational amplifier 70. In this manner, operational amplifier 70 operates with resistive feedback in an inverting mode to maintain a low input impedence, thereby operating photodiode 52 in a short circuit condition. As shown in FIG. 3, the input stage thereof preferably has a 10 to 1 gain variation control. Gain variation is achieved by adjusting the resistance of a suitable resistor 76 connected to the output of operational amplifier 70. By way of example, photodiode 52 may be a silicon wafer cell Model No. 88-22 manufactured by Solar Systems, Inc.

In order to detect the level at which toner particles are to be dispensed into the developer mix, comparator circuit 69 employs operational amplifier 78 wired as a level detector with a slight amount of positive feedback to provide hysteresis about the switch point. Resistor 80 is connected in parallel with operational amplifier 78, and preferably, has a resistance of 50,000 ohms.

The output from operational amplifier 70 is connected to the input of operational amplifier 78. A suitable voltage source (not shown) develops a l volt reference voltage to operational amplifier 78 through resistor 82, which is preferably 6,500 ohms. The positive terminal of operational amplifier 78 is connected to ground by resistor 84 which is, preferably, 1000 ohms. In this circuit, a voltage slightly more negative than 2.24 volts will cause the output of operational amplifier 78 to switch negative. The circuit holds that state until the input voltage is slightly less negative than 1.76 volts. The hysteresis cycle which is preferably about 2 volts 10.24 volts is useful to prevent high frequency oscillation at the switch point due to circuit instability and noise on the input signal. Temperature stability of this circuit is preferably about 0.5 micro volts/C over the total range of 20C. Thus, the total error will be about 10 microvolts over the total 20C range. Hence, it is evident that the major temperature sensitivity in comparator circuit 69 will be contributed by the thermal variations of the resistors. To reduce these to a minimum, metal film resistors are used which have a 0.01% per degree sensitivity.

Referring now to FIG. 4, hold electrical circuit 86 is depicted therein. Hold circuit 86 includes a voltage source preferably of volts (not shown) electrically connected in series to a suitable relay 88. Relay 88 is connected to a silicon controlled switch 90. The output from comparator circuit 69 is also connected to silicon controlled switch 90. This circuit detects the occurrence of a switch point and holds it until the logic is ready to interrogate it. The logic connection is shown at terminals 92. A detailed description of the logic associated with the toner dispensers is to be found in copending application Ser. No. 351,741 filed in 1973, now US. Pat. No. 3,873,002. Hold circuit 86 also converts the low level signal from the operational amplifier circuitry to the level required to dirve the toner dispenser motors. The toner dispenser motors are connected through logic lines 94. The reset of hold circuit 86 is accomplished by a signal from the logic circuitry to open relay 91.

Referring now to FIG. 6, there is shown a preferred circuit configuration for electrical circuitry 54 em bodying the various elements depicted in FIGS. 3 through 5, inclusive. As shown in FIG. 6, photodiode 52 is connected to operational amplifier 70. Resistor 72 and compacitor 74 are connected in parallel with operational amplifier 70. A 500,000 ohm resistor 94 is connected in series with the foregoing parallel arrangement and the output of operational amplifier 70. The output terminal of operational amplifier 70 is connected to a parallel circuit having three 5000 ohm resistors 96 in series with three 50,000 ohm variable resistors 98 connected to one another through switches 100. This circuit arrangement is connected to operational amplifier 78 of comparator 69. Operational amplifier 78 is connected to operational amplifier 70 and a 15 volt reference is applied thereto through resistor 82. The positive terminal of operational amplifier 78 is electrically grounded through resistor 84. Resistor 80 is connected in parallel with operational amplifier 78 as hereinbefore described. The output from operational amplifier 78 passes through resistor 102, which is preferably 2000 ohms. Resistor 102 is connected to ground through a 470 ohm resistor 104. Resistor 102 is also connected to silicon controlled switch 90. The anode of silicon controlled switch 90 is connected to relay 88 which, in turn, is connected to a 15 volt reference source. In addition, 100,000 ohm resistor 106 is connected from the anode gate of silicon controlled switch to the +15 volt voltage source. As previously indicated, the output along terminals 94 is directed to drive the toner dispenser oscillator motors. Interrogation of the hold circuit is initiated by a signal from the toner dispenser along logic lines 92.

In recapitulation, it is evident that the control apparatus of the present invention achieves the requisite regulation through the utilization of a thermally insensitive photosensor having a low level signal output therefrom which is amplified by a stable amplification circuit. The amplified electrical output signal is then compared to a reference by suitable comparator circuitry and then held in a hold circuit until interrogated by the toner dispenser logic circuitry. When the interrogated error signal is developed, it actuates the toner dispenser oscillator motor to dispense toner particles therefrom into the corresponding developer mix, thereby adjusting the concentration thereof to substantially about the preselected level. In this manner, the control apparatus insures that the image density and color balance of the multi-color copies being reproduced in the printing machine are repeatedly of a high quality.

It is, therefore, apparent that there has been provided in accordance with the present invention an apparatus for controlling the concentration of toner particles in a developer mix that fully satisfies the objects, aims and advantages set forth above. While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.

What is claimed:

1. An apparatus for controlling the concentration of particles in a mix, including:

electrode means biased electrically to attract particles thereto as said electrode means passes through the mix;

means for illuminating said electrode means having the particles deposited thereon with light rays;

a thermally insensitive photodiode for detecting the intensity of the light rays transmitted from said electrode means, said photodiode being adapted to generate a substantially thermally insensitive electrical output signal indicative of the density of particles deposited on said electrode means; and

amplifying circuit means, in electrical communication with said photodiode, for stably amplifying the thermally insensitive electrical output signal from said photodiode to at least a preselected level.

2. An apparatus as recited in claim 1, further including:

comparator circuit means, in electrical communication with said amplifying circuit means, for comparing the amplified electrical output signal with a preselected reference to produce a control signal corresponding to the deviation between the measured density of particles deposited on said electrode means and the desired density thereof; and

circuit means, in electrical communication with said comparator circuit means, for holding the control signal from said comparator circuit means.

9 3. An apparatus as recited in claim 2, further includmg:

means, actuated by the control signal from said hold circuit, for dispensing particles into the mix to achieve the requisite concentration therein.

4. An apparatus as recited in claim 3, wherein said illuminating means includes a light source.

5. An apparatus as recited in claim 4, wherein said amplifying circuit means includes:

a first operational amplifier connected electrically to said photodiode; and

a first resistance circuit connected in parallel with said first operational amplifier to provide resistive feedback operating in an inverting mode.

6. An apparatus as recited in claim 5, wherein said comparator circuit means includes:

a second operational amplifier in electrical communication with said first operational amplifier; and

a second resistance circuit connected in parallel with said second operational amplifier so that said second operational amplifier functions as a level detector with positive feedback about the switch point to provide a hysteresis cycle thereabout.

7. An apparatus as recited in claim 6, wherein said hold circuit means includes:

a silicon controlled switch in electrical communication with said second operational amplifier;

a relay connected electrically in series with said silicon controlled switch; and

a voltage source connected electrically in series with said relay.

8. An electrophotographic printing machine of the type having a photoconductive member and a development system employing a developer mix of carrier granules and toner particles to form a toner powder image on an electrostatic latent image recorded on the photoconductive member, including:

electrode means mounted on the photoconductive member and biased electrically to attract toner particles thereto as said electrode means passes through the developer mix;

means for illuminating said electrode means having the toner particles deposited thereon with light rays;

a thermally insensitive photodiode for detecting the intensity of the light rays transmitted from said electrode means, said photodiode being adapted to generate a substantially thermally insensitive electrical output signal indicative of the density of 10 toner particles deposited on said electrode means; and

amplifying circuit means, in electrical communication with said photodiode, for stably amplifying the thermally insensitive electrical output signal from said photodiode to at least a preselected level. 9. A printing machine as recited in claim 8, further including:

comparator circuit means, in electrical communication with said amplifying circuit means, for comparing the amplified electrical output with a preselected reference to produce a control signal corresponding to the deviation between the measured density of toner particles deposited on said electrode means and the desired density thereof; and

circuit means, in electrical communication with said comparator circuit means, for holding the control signal from said comparator circuit means.

10. A printing machine as recited in claim 9, further including means, actuated by the control signal from said hold circuit, for dispensing toner particles into the developer mix to achieve the requisite concentration therein.

11. A printing machine as recited in claim 10, wherein said illuminating means includes a light source,

12. A printing machine as recited in claim 11, wherein said amplifying circuit means includes:

a first operational amplifier connected electrically to said photodiode; and

a first resistance circuit connected in parallel with said first operational amplifier to provide resistive feedback operating in an inverting mode.

13. A printing machine as recited in claim 12, wherein said comparator circuit means includes:

a second operational amplifier in electrical communication with said first operational amplifier; and

a second resistance circuit connected in parallel with said second operational amplifier so that said second operational amplifier functions as a level detector with positive feedback about the switch point to provide a hysteresis cycle thereabout.

14. A printing machine as recited in claim 13, wherein said hold circuit means includes:

a silicon controlled switch in electrical communication with said second operational amplifier;

a relay connected in series with said silicon controlled switch; and i a voltage source connected electrically in series with said relay. 

1. An apparatus for controlling the concentration of particles in a mix, including: electrode means biased electrically to attract particles thereto as said electrode means passes through the mix; means for illuminating said electrode means having the particles deposited thereon with light rays; a thermally insensitive photodiode for detecting the intensity of the light rays transmitted from said electrode means, said photodiode being adapted to generate a substantially thermally insensitive electrical output signal indicative of the density of particles deposited on said electrode means; and amplifying circuit means, in electrical communication with said photodiode, for stably amplifying the thermally insensitive electrical output signal from said photodiode to at least a preselected level.
 2. An apparatus as recited in claim 1, further including: comparator circuit means, in electrical communication with said amplifying circuit means, for comparing the amplified electrical output signal with a preselected reference to produce a control signal corresponding to the deviation between the measured density of particles deposited on said electrode means and the desired density thereof; and circuit means, in electrical communication with said comparator circuit means, for holding the control signal from said comparator circuit means.
 3. An apparatus as recited in claim 2, further including: means, actuated by the control signal from said hold circuit, for dispensing particles into the mix to achieve the requisite concentration therein.
 4. An apparatus as recited in claim 3, wherein said illuminating means includes a light source.
 5. An apparatus as recited in claim 4, wherein said amplifying circuit means includes: a first operational amplifier connected electrically to said photodiode; and a first resistance circuit connected in parallel with said first operational amplifier to provide resistive feedback operating in an inverting mode.
 6. An apparatus as recited in claim 5, wherein said comparator circuit means includes: a second operational amplifier in electrical communication with said first operational amplifier; and a second resistance circuit connected in parallel with said second operational amplifier so that said second operational amplifier functions as a level detector with positive feedback about the switch point to provide a hysteresis cycle thereabout.
 7. An apparatus as recited in claim 6, wherein said hold circuit means includes: a silicon controlled switch in electrical communication with said second operational amplifier; a relay connected electrically in series with said silicon controlled switch; and a voltage source connected electrically in series with said relay.
 8. An electrophotographic printing machine of the type having a photoconductive member and a development system employing a developer mix of carrier granules and toner particles to form a toner powder image on an electrostatic latent image recorded on the photoconductive member, including: electrode means mounted on the photoconductive member and biased electrically to attract toner particles thereto as said electrode means passes through the developer mix; means for illuminating said electrode means having the toner particles deposited thereon with light rays; a thermally insensitive photodiode for detecting the intensity of the light rays transmitted from said electrode means, said photodiode being adapted to generate a substantially thermally insensitive electrical output signal indicative of the density of toner particles deposited on said electrode means; and amplifying circuit means, in elEctrical communication with said photodiode, for stably amplifying the thermally insensitive electrical output signal from said photodiode to at least a preselected level.
 9. A printing machine as recited in claim 8, further including: comparator circuit means, in electrical communication with said amplifying circuit means, for comparing the amplified electrical output with a preselected reference to produce a control signal corresponding to the deviation between the measured density of toner particles deposited on said electrode means and the desired density thereof; and circuit means, in electrical communication with said comparator circuit means, for holding the control signal from said comparator circuit means.
 10. A printing machine as recited in claim 9, further including means, actuated by the control signal from said hold circuit, for dispensing toner particles into the developer mix to achieve the requisite concentration therein.
 11. A printing machine as recited in claim 10, wherein said illuminating means includes a light source.
 12. A printing machine as recited in claim 11, wherein said amplifying circuit means includes: a first operational amplifier connected electrically to said photodiode; and a first resistance circuit connected in parallel with said first operational amplifier to provide resistive feedback operating in an inverting mode.
 13. A printing machine as recited in claim 12, wherein said comparator circuit means includes: a second operational amplifier in electrical communication with said first operational amplifier; and a second resistance circuit connected in parallel with said second operational amplifier so that said second operational amplifier functions as a level detector with positive feedback about the switch point to provide a hysteresis cycle thereabout.
 14. A printing machine as recited in claim 13, wherein said hold circuit means includes: a silicon controlled switch in electrical communication with said second operational amplifier; a relay connected in series with said silicon controlled switch; and a voltage source connected electrically in series with said relay. 